Multi-stage compressor and method of manufacturing a multi-stage compressor

ABSTRACT

A multi-stage compressor includes a first bore for a primary compression and a second bore for a secondary compression. A first piston is accommodated in the first bore and a second piston is accommodated in the second bore. A piston ring is attached to the first piston and a second piston ring is attached to the second piston. The inner diameters of the bores, the outer diameters of the pistons, and the outer diameters of the piston rings are equal to each other respectively. Thus, the bores are easily formed. Since only one type of piston and one type of piston ring are required to be manufactured, the manufacture of the above mentioned products is simplified. As a result, manufacture of the compressor becomes easy and the manufacturing cost is reduced.

TECHNICAL FIELD

[0001] The present invention relates to a multi-stage compressor for compressing fluid in compression chambers in multiple stages.

BACKGROUND ART

[0002] Japanese Unexamined Patent Publication No. 10-184539 discloses a typical two-stage compressor including a swash plate and single-headed pistons. According to this compressor, refrigerant gas is introduced into three first compression chambers from a suction chamber. The refrigerant gas is primarily compressed in the first compression chambers. The primarily compressed refrigerant gas is introduced into three second compression chambers. The primarily compressed refrigerant gas is secondarily compressed in the second compression chambers. The secondarily compressed refrigerant gas is discharged to a discharge chamber.

[0003] A cylinder block of the above mentioned compressor includes first bores forming the first compression chambers and second bores forming the second compression chambers. The inner diameters of the second bores are smaller than the inner diameters of the first bores. A first piston is accommodated in each first bore and a second piston is accommodated in each second bore. A first piston ring is attached to each first piston and a second piston ring is attached to each second piston.

[0004] The compression load and the theoretical volumetric efficiency of a multi-stage compressor change by less for a given change of compression ratio than those of a single-stage compressor. Thus, the multi-stage compressor is suitable for an air-conditioner.

[0005] The compressor disclosed in the above mentioned patent publication requires two types of bores with different diameters to be formed on the cylinder block. In addition to manufacturing two types of pistons with different diameters, two types of piston rings with different diameters must also be manufactured. In this case, the structure complicates the manufacturing process and thus increases the cost.

SUMMARY OF THE INVENTION

[0006] It is an objective of the present invention to provide an inexpensive multi-stage compressor that is easy to manufacture.

[0007] To achieve the above objective, the present invention provides a multi-stage compressor for compressing fluid at multiple stages. The compressor includes a plurality of bores for compression at multiple stages and a plurality of pistons, each of which is accommodated in one of the bore. The outer diameters of the pistons for compression of at least two different stages are the same.

[0008] Compared with a case in which the outer diameters of pistons are varied for different compression stages, the present invention requires fewer types of pistons to be manufactured. This facilitates the manufacture of the compressor and reduces the manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a cross-sectional view of a two-stage compressor according to a first embodiment of the present invention.

[0010]FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0011] A two-stage compressor 1 according to a first embodiment of the present invention will now be described with reference to FIG. 1 and FIG. 2. The compressor 1 is suitable for a freezer or an air conditioner. The compressor compresses a refrigerant gas such as carbon dioxide (CO₂) in two stages.

[0012] As shown in FIG. 1, the compressor 1 includes a cylinder block 3, a front housing member 2, a rear housing member 4, and a motor housing member 5. The front housing member 2 is fixed to the front end of the cylinder block 3. The rear housing member 4 is fixed to the rear end of the cylinder block 3. The motor housing member 5 is fixed to the front end of the front housing member 2. A seal ring 2 a is fitted between the motor housing member 5 and the front housing member 2. A seal ring 3 a is fitted between the front housing member 2 and the cylinder block 3. A seal ring 4 a is fitted between the cylinder block 3 and the rear housing member 4. A valve plate assembly 35 is located between the cylinder block 3 and the rear housing member 4. The left side of FIG. 1 is referred to as the front side of the compressor and the right side of FIG. 1 is referred to as the rear side of the compressor.

[0013] A motor 9 is accommodated in the motor housing member 5. A drive shaft 6 of the motor 9 extends from the motor housing member 5 to the cylinder block 3 through the front housing member 2. The front end of the drive shaft 6 is supported by the motor housing member 5 by means of a radial bearing 5 a. The rear end of the drive shaft 6 is supported by the cylinder block 3 by means of a radial bearing 6 a. The radial bearing 6 a is located inside a center bore formed in the cylinder block 3. A thrust race 8 a and a disc spring 8 b are located between the rear end face of the drive shaft 6 and the bottom wall of the center bore to urge the drive shaft 6 forward.

[0014] A drive chamber 33 is defined by the front housing member 2. A swash plate 7, which functions as a driving body or a drive plate, is fixed on the drive shaft 6 to be positioned inside the drive chamber 33. A thrust bearing 2 b is located between the swash plate 7 and the inner wall of the front housing member 2. The thrust bearing 2 b receives the force urging the drive shaft 6 and the swash plate 7 forward.

[0015] As shown in FIG. 1 and FIG. 2, the axes of the first bore 11 and the second bore 21 are parallel to the axis of the drive shaft 6. The bores 11 and 21 have equal inner diameters d1. Also, the bores 11 and 21 are located within a hypothetical circle, the center of which is on the axis of the drive shaft. The bores 11 and 21 are separated by 180 degrees angularly. The first piston 12 is accommodated in the first bore 11 and the second piston 22 is accommodated in the second bore 21. The first compression chamber 10, for primary compression, is formed in the first bore 11 between the head of the first piston 12 and the valve plate assembly 35. The second compression chamber 20, for secondary compression, is formed in the second bore 21 between the head of the second piston 22 and the valve plate assembly 35.

[0016] Each piston 12 and 22 is connected to the swash plate 7 by a pair of shoes 7 a. When the swash plate 7 integrally rotates with the drive shaft 6 in the direction indicated by an arrow 40 in FIG. 2, each piston 12 and 22 reciprocates in the corresponding bore 11 or 21. Arrows 42 and 44 in FIG. 1 show the moving direction of the second piston 22. The stroke of each piston 12 and 22 is determined by the inclination of the swash plate 7 with respect to a plane that is perpendicular to the axis of the drive shaft 6.

[0017] The first piston 12 has two ring grooves 12 a in its outer peripheral surface. The ring grooves 12 a are separated by a predetermined distance in the axial direction. A first set of piston rings 16 is fitted in the ring grooves 12 a. The second piston 22 has two ring grooves 22 a in its outer peripheral surface. The ring grooves 22 a are separated at a predetermined distance in the axial direction. A second set of piston rings 26 is fitted in the ring grooves 22 a. The first set of piston rings 16 and the second set of piston rings 26 are the same in size and have outer diameters d2 that are substantially equal to the inner diameters d1 of the bores 11 and 21. The pistons 12 and 22 also have outer diameters d2 that are substantially equal to the inner diameters d1 of the bores 11 and 21. Pistons 12, 22 and rings 16, 26 that differ in size only by manufacturing tolerances are considered to have “equal” dimensions in this specification.

[0018] When each piston 12 and 22 compresses refrigerant gas in the corresponding compression chamber 10 or 20, the piston rings 16 and 26 minimize leakage of refrigerant gas from the compression chambers 10 and 20 to the drive chamber 33. Thus, the compression efficiency is improved.

[0019] A suction chamber 30, an intermediate chamber 31, and a discharge chamber 32 are defined in the rear housing member 4. The rear housing member 4 has an inlet 4 b for receiving refrigerant gas from an external refrigerant circuit (not shown) to the suction chamber 30. The rear housing member 4 also has an outlet 4 c for discharging refrigerant gas from the discharge chamber 32 to the external refrigerant circuit. When the compressor is running, the pressure of the drive chamber 33 is substantially equal to the pressure of the suction chamber 30 (suction pressure Ps) and is lower than the pressure of the intermediate chamber 31 (intermediate pressure Pm) and the pressure of the discharge chamber 32 (discharge pressure Pd). The discharge pressure Pd is higher than the intermediate pressure Pm.

[0020] A first suction port 14 and a first discharge port 15, which correspond to the first compression chamber 10, and a second suction port 24 and a second discharge port 25, which correspond to the second compression chamber 10, are provided in the valve plate assembly 35. The first suction port 14 connects the suction chamber 30 to the first compression chamber 10. The first discharge port 15 connects the first compression chamber 10 to the intermediate chamber 31. The second suction port 24 connects the intermediate chamber 31 to the second compression chamber 20. The second discharge port 25 connects the second compression chamber 20 to the discharge chamber 32.

[0021] The valve plate assembly 35 includes a first suction valve 36, which corresponds to the first suction port 14, a first discharge valve 13, which corresponds to the first discharge port 15, a second suction valve 37, which corresponds to the second suction port 24, and a second discharge valve 23, which corresponds to the second discharge port 25.

[0022] The swash plate integrally rotates with the drive shaft 6 in the direction indicated by the arrow 40 in FIG. 2 when the drive shaft 6 is rotated by the drive motor 9. This reciprocates each piston 12 and 22 inside the corresponding bore 11 or 21. The pistons 12 and 22 are angularly separated by 180 degrees. Thus, when the first piston 12 is positioned at one of the top dead center or the bottom dead center, the second piston 22 is positioned at the other of the top dead center or the bottom dead center.

[0023] When the first piston 12 moves from the top dead center to the bottom dead center in the suction stroke, the refrigerant gas, which is under the suction pressure Ps, is drawn into the first compression chamber 10 through the first suction port 14 and the first suction valve 36. When the first piston 12 moves from the bottom dead center to the top dead center, in the compression/discharge stroke, the refrigerant gas is primarily compressed to the intermediate pressure Pm in the first compression chamber 10. The compressed refrigerant gas is then discharged to the intermediate chamber 31 through the first discharge port 15 and the first discharge valve 13.

[0024] When the second piston 22 performs the suction stroke, the refrigerant gas, which is under the intermediate pressure Pm, is drawn into the second compression chamber 20 through the second suction port 24 and past the second suction valve 37 from the intermediate chamber 31. When the second piston 22 is in the compression/discharge stroke, the refrigerant gas is secondarily compressed to the discharge pressure Pd in the second compression chamber 20. The secondarily compressed refrigerant gas is then discharged to the discharge chamber 32 through the second discharge port 25 and the second discharge valve 23.

[0025] As described above, the first compression chamber 10 and the second compression chamber 20 are formed in series with the intermediate chamber 31 in between. The pistons 12, 22 compress the refrigerant gas in the corresponding compression chambers 10, 20 at equal compression ratios, thus the refrigerant gas is compressed in two stages.

[0026] This embodiment provides following advantages.

[0027] The inner diameters of the two bores 11, 21 are the same, the outer diameters of the two pistons 12, 22 are the same, and the outer diameters of the piston rings 16, 26 are the same. Therefore, the bores 11, 21 are easily formed in the cylinder block 3. Also, only one type of piston and piston ring are required. This simplifies the manufacturing process. Furthermore, only one type of equipment is required for forming the bores 11, 21, pistons 12, 22, and piston rings 16, 26, respectively. As a result, the manufacture of the compressor is relatively simple and the manufacturing cost is reduced.

[0028] The sets of piston rings 16 and 26 fitted in the pistons 12, 22 effectively prevent the refrigerant gas from leaking to the drive chamber 33 from the compression chambers 10, 20.

[0029] When using carbon dioxide as the refrigerant gas, the difference between the suction pressure Ps and the discharge pressure Pd becomes equal to or greater than 5 MPa. Compared with the compressors using other refrigerant gas, the pressure of the compression chambers 10, 20 is relatively high. Accordingly, the gas tends to leak from the compression chamber 10, 20 easily. Thus, employing the piston rings 16, 26 to the pistons 12, 22 is preferred in the compressors that use carbon dioxide as the refrigerant gas.

[0030] The present invention may be changed as follows.

[0031] More than one first compression chamber 10 for the primary compression may be formed. In the same manner, more than one second compression chamber 20 may be formed.

[0032] The present invention may be applied to a compressor for compressing refrigerant in three stages or more. In this case, one or more piston may be used for each compression stage.

[0033] In a multi-stage compressor for compressing refrigerant in three or more stages using three or more pistons, the outer diameters of the pistons performing at least two different stages should be equal to each other. For example, in a multi-stage compressor for compressing refrigerant in N stages with N (N>4) pistons, the outer diameters of the pistons performing three of the compression stages (stage N, stage N-1, and stage N-2) may be t1. The outer diameters of the pistons performing two of the compression stages (stage N-3 and stage N-4) may be t2. The outer diameters of any other pistons may be t3.

[0034] The number and shape of the piston rings attached to the pistons 12, 22 may be altered arbitrarily. For example, helical piston rings may be used.

[0035] When the compressor is running, the difference between the pressure of drive chamber 33 and the second compression chamber 20 is greater than the difference between the pressure of the drive chamber 33 and the first compression chamber 10. Therefore, the refrigerant gas in the second compression chamber 20 leaks to the drive chamber 33 more easily than the refrigerant gas in the first compression chamber 10. Accordingly, the first set of piston rings 16 may be omitted.

[0036] As mentioned above, piston rings may be omitted as long as piston rings are located on the piston corresponding to the compression chamber that allows refrigerant gas to leak most easily. More specifically, piston rings may be used only on the piston of the compression chamber that differs the greatest in pressure from the pressure of the drive chamber 33. In other words, piston rings may be located only on the piston of the compression chamber that reaches the highest internal pressure.

[0037] In the compressor according to FIG. 1 and FIG. 2, the pressure of the drive chamber 33 is lower than the pressure of the first compression chamber 10 and the second compression chamber 20. However, the present invention may be applied to a compressor in which the pressure of the drive chamber 33 is substantially equal to the pressure of the second compression chamber 20. For example, the present invention may be applied to a compressor in which some of the compressed refrigerant gas is drawn from the second compression chamber 20 to the drive chamber 33. In such compressor, the refrigerant gas tends to leak to the first compression chamber 10 from the drive chamber 33. Therefore, it is preferred to fit piston rings on at least the first piston 12. In other words, the piston rings are used on the piston corresponding to the compression chamber with the lowest internal pressure.

[0038] Other fluid including ethylene (C₂H₄), ethane (C₂H₆), dyborane (B₂H₆), and liquefied nitrogen may be used as the refrigerant gas instead of the carbon dioxide. 

1. A multi-stage compressor for compressing fluid in multiple stages, comprising: a plurality of bores for compression at multiple stages; and a plurality of pistons, each of which is accommodated in one of the bores, wherein the multi-stage compressor is characterized in that the outer diameters of the pistons for compression of at least two different stages are the same.
 2. The multi-stage compressor according to claim 1, characterized in that the outer diameters of all pistons for compression at multiple stages are the same.
 3. The multi-stage compressor according to claims 1 or 2, characterized by a drive chamber accommodating a driving body for driving the pistons, wherein a piston ring is attached at least to one of the pistons corresponding to one of the bores that has the greatest difference between the internal pressure and the pressure of the drive chamber.
 4. The multi-stage compressor according to claims 1 or 2, characterized in that a piston ring is attached at least to one of the pistons corresponding to one of the bores that has the greatest internal pressure.
 5. The multi-stage compressor according to claims 1 or 2, characterized in that a piston ring is attached at least to one of the pistons corresponding to one of the bores that has the lowest internal pressure.
 6. The multi-stage compressor according to any one of claims 1 to 5, characterized in that the fluid is carbon dioxide. 